The invention relates to a process of regenerating spent hydrocarbon conversion catalyst by the combustion of coke on the catalyst in a fluidized combustion zone. This invention specifically relates to a process for the conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke that acts to deactivate the catalyst.
Fluid catalytic cracking (FCC) is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke is deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone.
A common objective of these configurations is maximizing product yield from the reactor while minimizing operating and equipment costs. Optimization of feedstock conversion ordinarily requires essentially complete removal of coke from the catalyst. This essentially complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 wt-% coke. In order to obtain complete regeneration, the catalyst has to be in contact with oxygen for sufficient residence time to permit thorough combustion.
Conventional regenerators typically include a vessel having a spent catalyst inlet, a regenerated catalyst outlet and a distributor for supplying air to the bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the spent combustion gas before the gas exits the regenerator vessel. In a dense catalyst bed, also known as a bubbling bed, combustion gas forms bubbles that ascend through a discernible top surface of a dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed.
One way to obtain fully regenerated catalyst is by performing the regeneration in stages. The use of relatively dilute phase regeneration zones to effect complete catalyst regeneration is shown in U.S. Pat. Nos. 4,430,201; 3,844,973 and 3,923,686. These patents teach a lower dense bed to which combustion gas is distributed and an upper transport zone. A two-stage system that combines a relatively dilute phase transport zone without a lower dense bed zone for regenerating catalyst is shown in U.S. Pat. Nos. 5,158,919 and 4,272,402. These patents all teach an upper dense bed into which the at least partially regenerated catalyst exiting from the transport zone collects. U.S. Pat. Nos. 4,197,189 and 4,336,160 teach a riser combustion zone in which fast fluidized flow conditions are maintained to effect complete combustion without the need for the additional combustion in the catalyst bed collected from the top of the riser.
In regenerators that have two or more chambers typically separated by a riser section, a riser termination device may be used to roughly separate most of the at least partially regenerated catalyst from the flue gas that is generated upon combustion of coke deposits. A tee disengager is a riser termination device that has one or more arms extending from and in downstream communication with the riser. An opening in the arm discharges regenerated catalyst and flue gas downwardly to roughly separate the descending heavier catalyst from the lighter flue gas that tends to ascend in a second or typically, upper chamber. An example of a tee disengager is shown in U.S. Pat. No. 5,800,697.
Another type of riser termination device used on FCC reactors comprises two or more tubes which extend from an opening in the riser and turn downwardly. Regenerated catalyst and product gases exit an opening in the end of the tube discharging downwardly. Examples of such riser termination devices are in U.S. Pat. Nos. 4,397,738; 4,482,451; 4,581,205 and 4,689,206.
As greater demands are placed on FCC units, regenerator vessels are being required to handle greater catalyst throughput. Greater quantities of combustion gas are added to the regenerator vessels to combust greater quantities of catalyst. As combustion gas flow rates are increased, so does the flow rate of catalyst exiting the riser termination device increase.